Means for limiting the modulation of the output of transmitters of communication systems



Dec. 20, 1949 A. J. SORENSEN MEANS FOR LIMITING THE MODULATION OF THEOUTPUT OF TRANSMITTERS OF COMMUNICATION SYSTEMS 3 Sheets-Sheet 1 FiledSept. 4, 1947 INVENTOR. 4nd' ew 50119125912 His Arrow w QMR w NW2 Dec.20, 1949 A. J. SORENSEN 2,491,590 MEANS FOR LIMITING THE MODULATION OFTHE OUTPUT OF TRANSMITTERS OF COMMUNICATION SYSTEMS 3 Sheets-Sheet 2Filed Sept. 4, 1947 QM V NQQ E i gm INVENTOR.

sag QR {N 8% EN QSS Q Patented Dec. 20, 1949 MEANS FOR LIMITING THEMODULATION OF THE OUTPUT OF TRANSMITTERS OF COMMUNICATION SYSTEMS AndrewJ. Sorensen, Edgewood, Pa.., assignor to The Union Switch & SignalCompany, Swissvale, Pa., a corporation of Pennsylvania ApplicationSeptember 4, 1947, Serial No. 772,140

My invention relates to means for limiting the modulation of the outputof transmitters of communication systems, and more particularly to meansfor limiting the modulation of the output of transmitters of carriertelephone systems.

The transmitters of many communication systems must function underwidely different conditions. For example, in railway train carriertelephone systems the transmitter must at times operate on a locomotivewhere high background noise prevails and extreme bursts of noise occur.Also at such locations there is a tendency for the operator to speak ina loud voice and close to the microphone when telephoning. At otherlocations, such as a wayside office, the ambient noise may be relativelylow and the operator may often be some distance away from the microphoneand there may be a tendency for him to speak in a low voice.

Furthermore, the circuit network for connecting the microphone to themodulator of the transmitter generally includes capacitance andinductance and the impedance of the circuit network is subject toconsiderable variation with frequency. This results in resonantconditions occurring at certain frequencies giving a large undesiredresponse at these particular frequencies.

Again, carrier telephone systems of the type here contemplated operateon an assigned frequency channel and it is necessary to prevent thetransmitter from producing frequencies extending into the adjacentchannel which would otherwise cause undesired response in the systemassigned to the adjacent channel.

Accordingly, a feature of my invention is the provision of novel andimproved means for limiting the modulation of the output of transmittersof carrier communication systems.

Another feature of my invention is the provision of novel and improvedmeans to secure a more nearly uniform output of a transmitter of acarrier telephone system for loud and low voices.

Again, a feature of my invention is the provision of improved means toreduce the effects of high noise level and violent bursts of noise ontransmitters of carrier telephone systems.

Still another feature of my invention is the provision of a carriertelephone system transmitter incorporating improved means to minimizethe effects of resonance in the microphone circuit network.

A more specific feature of my invention is the provision of improvedmeans for limiting the 5 Claims. (Cl. 332-19) 2 modulation of thecarrier of a railway train inductive carrier telephone system usingfrequency modulation, and thereby secure a more nearly uniform frequencydeviation of the carrier from its designated center frequency for loudand low voices.

Other features, objects and advantages of my invention will appear asthe specification progresses.

The foregoing features, objects and advantages of my invention I attainby providing a. transmitter incorporating a modulation limiter in acircuit network connecting the microphone to the modulator. This circuitnetwork includes a microphone transformer and its usual capacitance andinductance and the modulation limiter, the limiter being interposed inthe network in either the primary or the secondary side of themicrophone transformer. The modulation limiter comprises two asymmetricunits connected in parallel opposition. These asymmetric units arecharacterized by a forward direction resistance that is a function ofthe applied voltage. That is to say, the resistance of each asymmetricunit decreases as the applied voltage increases. Preferably, theasymmetric units are of a dry surface contact type and may be, forexample, a stack of copper oxide rectifier discs or elements. Eachasymmetric unit is preselected as to the number of discs it includes sothat the stack of discs is proportioned to agree with the voltage of thecircuit in which the unit is connected. Also, the rectifier elements areselected as to their size for the limiter to impedance match the primaryor secondary winding of the microphone transformer according as thelimiter is interposed in the network on the primary or secondary side ofthe transformer.

By such proportioning, the limiter has almost uniform impedance over theessential part of the voice frequency band, but at the low frequencies,such as, 200 cycles and below, and at frequencies above a given value,such as 3000 cycles, the limiter acts as an effective band pass filter.1 In this way the modulation limiter secures a more uniform response forloud and low voices, and extremely loud voices will not overload thetransmitter to the extent that would be possible without the limiter.

Under very noisy conditions, the effects of the high noise level will beconsiderably minimized because the noise which always fills in the gapsin the conversation will be materially reduced. Also, violent bursts ofnoise will not be able to operate the .modulator of! its usual range ofoperation and interference and distortion due to over modulation will bereduced. Furthermore, the tendency'toward resonance in the microphonecircuit network will be either suppressed or greatly minimized.

I shall describe two forms of means for limiting the modulation of theoutput of transmitters embodying my invention and shall then point outthe novel features thereof in claims.

In the accompanying drawings, Figs. 1 and 2 are diagrammatic viewsshowing a first and a second form, respectively, of a transmitterincorporating modulation limiting means embodying my invention, thetransmitter being that for a carrier telephone system using frequencymodulation. Figs. 3, 4. and are diagrams showing differentcharacteristics of the-modulation limiting means of Figs. 1 and 2.

In each of the several views like reference characters are usedto'designate similar parts.

It is to be understood that the invention is not confined to atransmitter using frequency modulation and this one applicationillustrates the many places the apparatus is usefulv Referring to Fig.1, the transmitter includes a microphone, a modulation limiter, amodulator,.

an oscillator, and a power source as the essential elements.

The power source may be any convenient arrangement and as here disclosedit includes a low voltage source and a motor generator for converting alow voltage into a voltage suitable for energizing the anode circuits ofthe electron tubes of the transmitter. The low voltage source may be a32 volt battery, not shown, the terminals of which are indicated at X32and N32. A motor generator MG has its motor I0 connected across theterminals X32 and N32 for operation thereof. The generator ll of themotor generator supplies a relatively high direct voltage, such as 300volts, the terminals of the generator being indicated as B300 and N300.Following the usual custom, the positive terminal X32 of the low voltagesource and the negative terminal N300 of the generator are connected toa ground electrode l2. g

The oscillator is preferably ofthe electron tube type, and as heredisclosed it includes an oscillating circuit 0C and an electron tube VI.The oscillating circuit 00 is of a standard form of a parallel connectedinductance l3 and capacitance H. The tube VI is a pentode but othertubes can be used. The anode-cathode circuit of tube VI is powered fromgenerator H and the oscillating circuit 0C is connected to the tube in awell-known arrangement as will be apparent by an inspection of Fig. l.The parts are so proportioned that oscillations of a definite carrierfrequency are produced, and

as an aid in understanding the invention I shall assume that theoscillator supplies a carrier current of 100 kc. but other carrierfrequencies can be provided.

The'output of this oscillator will be connected to any suitable form oftransmitting circuit and in Fig. 1 the oscillator is shown coupled to atransmitting circuit through a driver stage DS and a power amplifier PA,the output of the power amplifier including an output transformer T2.The driver stage DS and power amplifier PA are shown in block formbecause these devices can be of star tlard arrangements for such devicesand they are not essential elements of the invention. I It is apparentthat if corresponding variations of the oscillations sup-, plied by theoscillator.

additional amplification of the communication currentis not required,the driver stage D8 and power amplifier PA may be omitted.

The modulator of the. transmitter is vof the reactance tube form andincludes an electron tube V2, the tube V2 being here shown as a tetrodebut other tubes can be employed. The tube V2 is providedwith ananode-cathode circuit which extends from terminal B300 of generator llthrough resistors I5 and I6, inductance coil l1, anode l8 and tube spaceto cathode l3 of tube V2, a biasing unit BU and ground electrodes and I2to negative terminal N300 of the generator. This anode-cathode circuitis coupled to the oscillating circuit of the oscillator, the anode oftube V2 being connected to one terminal of the circuit-0C through acoupling capacitor 22 and the cathode side of the anodecathode circuitbeing connected to the other terminal of the oscillating circuit throughground electrodes 20 and 9. Also the circuit 00 is connected to controlgrid 23 of tube V2 through resistor 24 and capacitor 25. It is apparentthat the tube V2 is in parallel with the oscillating circuit and thereactance of the tube V2 will vary the oscillating circuit and causeThus it is seen that voltages applied to control grid 23 of tube V2 tovary the reactance of the tube will serve to frequency modulate thecarrier supplied by the oscillator.

A microphone M is connected to the input of wire 26, microphone M, wire21, primary wind ing 28 of transformer Tl, inductance coil 29 andresistor 30 to terminal N32 of the current source, a capacitor 3| beingconnected across the microphone and a capacitor 32 being connectedbetween the terminal of primary winding 28 remote from the microphone,and ground. A secondary winding 33 of the microphone transformer TI istuned by a capacitor 31 and it has one terminal connected to controlgrid 23 of the modulator tube V2 through a resistor 34 and an inductancecoil 35 and its other terminal connected to ground, a resistor 36 beingconnected between the junction terminal of resistor 34 and coil 35 andground. It follows that audio frequencies impressed upon the microphoneM cause corresponding variations in the current flowing in themicrophone circuit and these current variations induce a correspondingelectromotive force in secondary winding 33 of the transformer, and thiselectromotive force is impressed upon the control grid 23 of themodulator tube V2 to vary the reactance of the tube and frequencymodulate the carrier supplied by the oscillator. That is, noise energyimpressed upon the microphone and an operator speaking into themicrophone cause corresponding frequency modulation of the carrier.

As is well known in frequency modulation systems, the amplitude of themodulating energy determines the frequency deviation of the carrier fromits designated center frequency. Thus, with a given assigned frequencyband of say, for example, plus or minus 3 kc., for the transmitter hereinvolved, a voltage of a magnitude greater than a given value applied tothe modulator tube V2 will over-modulate the carrier and cause afrequency deviation beyond the assigned band.

The modulation limiter ML comprises two asymmetric units 38 and 39connected in parallel opposition. In Fig. 1 the limiter ML is connectedacross the primary winding 28 of the microphone asymmetric units arecharacterized by a resistance in their forward direction that is afunction of the voltage applied. This characteristic feature isillustrated by the curve in Fig. 3, the resistance of the unitdecreasing rapidly with an increase in the voltage applied thereto for agiven range of voltages. The units 38 and 39 are made up of a stack ofrectifier elements, the number of elements used in the stack beingdetermined by the voltage appearing across the primary winding 28 of themicrophone transformer. In practicing the invention I have found thatthe voltage appearing across the primary winding of the microphonetransformer is relatively low and a stack of copper oxide rectifierelements of six discs is usually satisfactory. The size of the rectifierelements or discs is selected so that the limiter ML impedance matchesthe primary winding of the transformer.

Due to the capacitance and inductance involved in this microphonecircuit network, the network approaches resonance at certain audiofrequencies and undesired responses are effected at such frequencies. Ihave found that for a microphone circuit network of a transmitter thatis in commercial use, the voltage applied to the reactance tube of thetransmitter varies substantially according to a curve A of Fig. 5 foraudio frequencies when the modulation limiter ML is not used. It is tobe noted from curve A that at a low audio frequency of the order of 200cycles per second a resonant condition occurs and a very high voltage isapplied to the reactance tube. This means that voice frequencies andnoise energy frequencies of the order of 200 cycles create a response inthe oscillator that is all out of proportion to the importance of thesefrequencies. Again, as shown .by curve A, a resonant condition isapproached and a high value of voltage applied to the reactance tube forfrequencies over 2500 cycles. is an undesired response to voice andnoise energy frequencies of the order of 2500 cycles. Thus withtransmitters as heretofore uscd an unde sired response is effected tovery low frequencies and also at the upper limit of the voice frequencyband desired, the usual Voice frequency band be-' ing taken as extendingfrom 400 to 2500 cycles in systems of the type here involved.

With the modulator limiter ML connected into the circuit network, thevoltage appearing at the modulator tube for different audio frequenciesis substantially that illustrated by the curve B of Fig. 5. It is to beobserved' from curve B that with the modulator limiter the high voltagepeak of the low audio frequency of 200 cycles is entirely suppressed andthe high response of the higher audio frequency of the order of 2000 to2500 cycles is greatly reduced. In fact the curve B shows that theresponse for energy between 400 and 2500 cycles is substantiallyuniform, or at least it is much more nearly un form than that obtainedwithout the use of the modulation limiter.

Again, in Fig. 4, the curve C shows the variations of the voltageapplied to the modulator tube In other words, there 2,1

for different values or amplitudes of the input voltage, that is, thevoltage created in the microphone circuit by speaking into themicrophone when the modulator limiter ML is not used. The curve D showsthis operating characteristic of the microphone circuit network when themodulator limiter ML is used. It is to be noted from curves C and D thatthe voltages applied to the modulator tube increase much less between anormal voice and a very loud voice when the modulation limiter is used.That is, the response of a transmitter is much more nearly uniformbetween loud and low voices when the modulator limiter is connected intothe microphone circuit network. In other words, over-modulation is re-'duced by use of the modulation limiter.

Referring to Fig. 1, it is also to be noted that a blocking capacitormay be connected in series with the modulation limiter ML to block theflow of direct current of the microphone circuit through the limiter,but I have found that under the usual condition the direct voltage dropacross the primary winding 28 of the microphone transformer is low and ablocking capacitor .in series with the modulation limiter is notrequired.

In Fig. 2, the transmitter is substantially the same as that in Fig. 1except a modulation limiter MLI is connected across the secondarywinding 33 of the microphone transformer Tl in place of the limiterbeing connected across the primary winding as shown in Fig. 1. Thelimiter MLI is composed of two asymmetric units 43 and 4| connected inparallel opposition the same as the asymmetric units of the limiter MLof Fig. 1. Ordinarily, the voltage across the secondary winding of themicrophone transmitter is higher than the voltage across the primarywinding and thus the units 40 and 4| of the limiter MLI are made up of agreater number of rectifier discs to agree with this higher voltage ofthe secondary winding. Also, the discs are of a smaller size for thelimiter MLI to impedance match the secondary winding 33 of themicrophone transformer. When thus constructed the modulator limiter MLIprovides the transmitter of Fig.2 with operating characteristicssubstantially the same as those obtained for the transmitter of Fig. 1by the modulation limiter ML.

In practicing the invention, I have found that transmitting apparatushere disclosed when used with a railway train inductive carriertelephone system not only has the advantages of assuring a nearlyuniform response for loud and low voices but also the effects ofbackground noise and bursts of noise are minimized giving a higher orderof intelligibility of the transmitted speech. Also, over-swinging orover-modulation of the carrier frequency beyond the assigned band isavoided. Furthermore, by proper choice of the modulation limiter, thetransmitter is an unexpected improvement due to the suppression of theresonant conditions that ordinarily occur in the microphone network whenthe modulation limiter is not used.

Although I have herein shown and described but two forms of means forlmiting the modulation of the output of transmitters of communicationsystems embodying my invention, it is understood that various changesand modifications may be made therein within the scope of the appendedclaims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In a transmitter for a. carrier telephone 7 system, the combinationcomprising, an electron tube modulator having connections to a source ofcarrier current of a definite frequency for modulating the carriercurrent according to voltage variations applied to a. control electrodeof the modulator tube, a microphone, a transformer, a first circuitmeans including capacitance and inductance to connect said microphone toa primary winding of said transformer, a second circuit means includingcapacitance and inductance to connect a secondary winding of said transformer to said control electrode and a cathode of said tube, amodulation limiter comprising a pair of asymmetric units connected inparallel opposition, said asymmetric units characterized by an impedancethat is a function of the voltage applied thereto, said modulationlimiter connected across said primary winding of said transformer tolimit the magnitude of, the voltages appearing across said primarywinding for limiting the magnitude of the voltages applied to'saidmodulator tube.

'2. In a transmitter for a carrier telephone system, the combinationcomprising, an electron tube modulator having connections to a source ofcar.- rier current of a definite frequency for modulating the carriercurrent according to voltage variations applied to a control electrodeof the modulator tube, a microphone, a transformer, a first circuitmeans including capacitance and inductance to connect said microphone toa primary winding of said transformer, a second circuit means includingcapacitance and inductanceto connect a secondary winding of saidtransformer to said control electrode and a cathode of said tube, amodulation limiter comprising a pair of asymmetric units connected inparallel opposition, said limiter connected across said primary winding,and said asymmetric units having a forward direction characteristic of adecreasing impedance with an increase of applied voltage andproportioned to impedance match said primary winding to modify thevoltage applied to said tube due to ,loud speaking into said microphoneand to resonant conditions of said first circuit means.

3. In a.transmitter for a carrier telephone system using frequencymodulation, the combination comprising, an electron tube oscillator forsupplying a carrier of a definite center frequency, a reactancemodulator tube connected to said oscillator to frequency modulate saidcarrier according to voltages applied to a control electrode of saidmodulator tube, a microphone, a transformer, a first circuit meansincluding a source of current to connect said microphone across aprimary winding of said transformer, a second circult means to connect asecondary winding of said transformer to said control electrode of saidmodulator tube to apply to said control electrode j voltages created byaudio frequencies impressed on said microphone, a modulation limitercomprising two asymmetric units arranged in parallel opposition, saidunits characterized by an impedance which varies with the voltageapplied thereto, and said limiter connected across said first circuitmeans and proportioned to limit the voltage applied to said primarywinding and to eliminate resonant conditions of the first circuit meansat voice frequencies to confine the frequency deviation of said carrierto a given value each side of the center frequency.

4. In a transmitter for a carrier telephone sysprimary winding of saidtransformer, a second circuit means to connect a secondary winding ofsaid transformer to said control electrode of said modulator tube toapply to said control electrode voltages created by audio frequenciesimpressed on said microphone, a modulation limiter comprising twoasymmetric units arranged in parallel opposition, said unitscharacterized by an impedance which varies with the voltage appliedthereto, and said limiter connected across said second circuit means andproportioned to govern the voltage applied to said modulator tube to agiven magnitude.

5. In a transmitter for a carrier telephone system using frequencymodulation,'the combination comprising; an oscillator including anelectron tube having an anode, a cathode and a control electrode and anoscillating circuit having capacitance and inductance; said oscillatormade operable to supply a carrier of a definite center frequency; areactance modulator tube having an anode, a cathode and a controlelectrode; said modulator tube having an anode-cathode circuit coupledacross said oscillating circuit to frequency modulate said carrieraccording to voltage variations applied to said control electrode andcathode of said modulator tube, a microphone, a transformer, circuitmeansincluding a source of current to connect said microphone across awinding of said transformer, another circuit meansto couple anotherwinding of said transformer to said control electrode and cathode ofsaid modulator tube to apply to the control electrode voltages createdby audio frequencies impressed on said microphone, a. modulation limitercomprising two electrical asymmetric units of the dry surface contacttype arranged in parallel opposition, said modulation limiter connectedacross a selected one of said transformer windings to control thevoltages applied to the control electrode of the reactance modulatortube, said asymmetric units having a forward direction characteristic ofa rapidly decreasing impedance for a given range of applied voltages tolimit the voltages applied to the modulator tube due to loud speakinginto the microphone, and said asymmetric units selected as to their sizefor their elements to have a given impedance to impedance match thewinding to which the limiter is connected to minimize resonant conditioneffects of said first mentioned circuit means.

ANDREW .I. SORENSEN.

REFERENCES CITED The following references are of record in the file ofthis patent: V

V UNITED STATES PATENTS Number

